The present invention is intended to impart durable superoleophobic characteristics to a wide variety of metal, ceramic, or polymeric substrates by a rapid and simple process.
Fuels, oils, and greases typically possess a surface tension of 20-35 mJ per square meter, which is lower than almost all solid surfaces in which these fluids are in contact. Thus, fuels, oils, and greases tend to spread on almost all types of surfaces. Even when fuels, oils, and greases do not spread on surfaces, they can be difficult to remove due to the adhesion of individual liquid droplets. However, surfaces having a sufficiently low energy combined with the appropriate geometric texture involving re-entrant curvature or roughness at multiple (small) length scales can minimize the adhesion by forming composite liquid/air interfaces in contact with the surface. In such interfaces, only a portion of the solid surface is in direct contact with the liquid; the rest of the solid surface is covered by pockets of air or vapor. The partial nature of the solid/liquid contact reduces the force of adhesion and is most conveniently demonstrated by measuring the amount of vertical tilt needed to cause a droplet to roll off the surface rather than adhere. In conventional rough surfaces that are resistant to the spreading of fuels, oils, and greases, these angles tend to be large, however, when a composite interface is present, a tilt angle of less than about 20 degrees, is sufficient to cause droplets to roll off. When a surface tends to form composite air/liquid interfaces with fuels, oils, and greases, characterized by very high contact angles (often over 120 degrees), with only a small difference between advancing and receding contact angles, and with roll-off angles less than about 20 degrees, the surface is said to be “superoleophobic”. Although the phenomenon of superoleophobicity has been described for many decades, it is only since about 2007 that a significant amount of attention has been paid to it. As a result, there is as yet no commonly accepted precise definition of “superoleophobic”, though the working definition as noted above suffices for most practical situations.
Superoleophobic surfaces are in general extremely rare and thus far unknown in nature, though surfaces exhibiting analogous behavior towards water, known as “superhydrophobic” surfaces, are widely known. Despite the rarity of superoleophobic surfaces, the easy cleaning and resistance to spreading by fuels, oils, and greases imparted by superoleophobicity represents a highly desirable property for many surfaces, not only for items such as engine components, fuel system components, and transparencies found in military applications, but for a number of items in widespread use such as touchscreen displays, food handling equipment, and clothing. There is thus a widespread need for superoleophobic surface technology; in particular, there is a need for a coating formulation that, when applied to any underlying surface by a simple method, imparts superoleophobicity, and remains in-tact after exposure to elevated temperatures. There is a further need for such a coating to be readily prepared in a variety of different formulations having a controlled and easily tuned degree of superoleophobicity in concert with a controlled and easily tuned degree of hardness, durability, and optical characteristics. Such a coating can be said to exhibit “robustly designed superoleophobicity” in the sense that industrial product designers have the freedom to alter the formulation in simple ways without destroying the superoleophobic character of the surface, and that a small change in any given formulation variable can be expected to lead to a small change in the degree of superoleophobicity, which may be determined by the range of fluids that will form composite interfaces, or more quantitatively, by the lowest surface tension that a liquid can possess and still form a composite interface.
As mentioned, the known examples of superoleophobic surfaces are few. For instance, complex processes such as chemical vapor deposition, electrodeposition, lithography, etching, electrospinning, or sol-gel processing have been used to fabricate such surfaces (U.S. Pat. No. 7,455,911; U.S. Pat. No. 6,383,642; A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella, G. C. Rutledge, G. H. McKinley and R. E. Cohen, Science, 2007, 318, 1618; A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley and R. E. Cohen, Proc. Natl. Acad. Sci. U.S.A., 2008, 105, 18200; A. Tuteja, W. Choi, G. H. McKinley, R. E. Cohen and M. F. Rubner, MRS Bull., 2008, 33, 752; T. Darmanin and F. Guittard, J. Am. Chem. Soc., 2009, 131, 7928; Y. Liu, Y. Xiu, D. W. Hess, and C. P. Wong, Langmuir, 2010, 26, 8908). Such processes, however, do not allow for the facile coating of a wide variety of substrates.
Simpler processes, such as dip coating, have been used to impart superoleophobic behavior to common fabrics and biological surfaces (W. Choi, A. Tuteja, S. Chhatre, J. M. Mabry, R. E. Cohen and G. H. McKinley, Adv. Mater., 2009, 21, 2190; S. S. Chhatre, W. Choi, A. Tuteja, K.-C. Park, J. M. Mabry, G. H. McKinley and R. E. Cohen, Langmuir, 2010, 26, 4027; S. S. Chhatre, A. Tuteja, W. Choi, A. Revaux, D. Smith, J. M. Mabry, G. H. McKinley and R. E. Cohen, Langmuir, 2009, 25, 13625). The coatings, however, are easily removed on contact with any liquid and can only impart superoleophobicity to substrates with specific pre-existing textures. Spin coating has also been used to form fragile superoleophobic surfaces (Y. C. Sheen, Y. C. Huang, C. S. Liao, H. Y. Chou, and F. C. Chang, J. Polym. Sci. Polym. Phys. Ed., 2008, 46, 1984), but again the range of substrates that can be rendered superoleophobic via this technique is limited.
Despite its simplicity and applicability to any substrate, spray coating has not yet been widely used to create superoleophobic surfaces. In the lone published instance where it has been employed (A. Steele, I. Bayer and E. Loth, Nano Lett., 2009, 9, 501), the conditions leading to superoleophobicity were extremely narrowly defined, precluding “robustly designed” coatings, and the maximum use temperature of the fluoro-acrylate materials employed was limited to about 125° C. The present invention, however, relates to a spray coated superoleophobic surface that survives exposure to temperatures as high as 200° C. and that possesses “robustly designed” superoleophobicity due to the gradual alteration of its ability to form composite interfaces with changes in composition.